1. Field of the Inventions
The present inventions relate to method and apparatus for preparing thin films of semiconductor films for radiation detector and photovoltaic applications.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors are excellent absorber materials for thin film solar cell structures. Group IBIIIA VIA compound semiconductors includes some of: the Group IB elements of the periodic table such as copper (Cu), silver (Ag), and gold (Au); the Group IIIA elements of the periodic table such as boron (B), aluminum (Al), gallium (Ga), indium (In), and (Tl); and, the Group VIA elements of the periodic table such as oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po). Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1−xGax (SySe1−y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications. Alkali metals of Group IA, such as K, Na and Li are often included in the CIGS(S) absorbers as dopants to improve their photovoltaic properties.
The structure of a conventional Group IBIIIA VIA compound photovoltaic cell 10 such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1. The photovoltaic cell 10 includes a base 11 having a substrate 12 and a conductive layer 13 formed on the substrate. The substrate 12 can be a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. An absorber thin film 14, which includes a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is formed on the conductive layer 13. The conductive layer can be a Mo, Ta, W, or Ti layer, and functions as an ohmic contact to the photovoltaic cell. However, if the substrate 12 is a properly selected conductive material such as Ta foil or Mo foil, it is also possible not to use a conductive layer, since the substrate 12 can be used as an ohmic contact to the photovoltaic cell. After the absorber film 14 is formed, a transparent layer 15, for example, a CdS, ZnO or CdS/ZnO film stack is formed on the absorber film. Light 16 enters the photovoltaic cell 10 through the transparent layer 15. Metallic grids (not shown) are formed over the transparent layer 15 to reduce the effective series resistance of the device. The preferred electrical type of the absorber film 14 is p-type, and the preferred electrical type of the transparent layer 15 is n-type. However, an n-type absorber and a p-type window layer can also be formed. The device structure shown FIG. 1 is called a substrate-type structure. A so called superstrate-type structure can also be formed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te)2 absorber film, and finally forming an ohmic contact to the device by a conductive layer. In the superstrate structure light enters the device from the transparent superstrate side.
One technique for growing Cu(In,Ga)(S,Se)2 type absorber thin films for solar cell applications is a two-stage process where metallic components or constituents of the Cu(In,Ga)(S,Se)2 material, i.e. Cu, In and Ga, are first deposited onto a substrate, and then reacted with the non-metallic constituents (or semi-metallic constituents), i.e. S and/or Se, in a high temperature annealing process. Alternatively, Group VIA material layers can be also included in the precursor. For example, Se and/or S can be deposited over a stack of Cu, In and/or Ga films, and this precursor stack is annealed at elevated temperatures (400-600° C.) to initiate reaction between the metallic elements and the Group VIA material(s) to form the Cu(In,Ga)(S,Se)2 compound. During the annealing, additional Se and/or S sources, such as Se and S vapors, or Se and S containing gases can be also delivered into the reactor. Selenium vapor migration to adjacent stations and equipment is an important problem in deposition systems using Se evaporators. In such systems, during the deposition, selenium vapor migrates to adjacent deposition stations or end stations and the mechanisms such as winding mechanisms in the end stations or various rollers supporting the web during the deposition. One prior art solution is to increase free-span distance of the web to move adjacent stations and winding mechanisms further away from selenium evaporators. However there are drawbacks with this solution, because a longer span causes the web to droop with a catenary shape, which results in degradation of uniformity in the depositing layer due to uneven tension. Such large free span also increases system footprint. The absorber layer 14 shown in FIG. 1 may contain dopant elements, such as Na to enhance cell performance, in addition to the primary elements (Cu, In, Ga, Se and/or S) required to form the absorber layer. Prior research on possible dopants for Group IBIIIA VIA absorber layers has shown that alkali metals, such as Na, K, and Li, affect the structural and electrical properties of such absorber layers. Especially, inclusion of Na in CIGS layers was shown to be beneficial for their structural and electrical properties and for increasing the conversion efficiencies of solar cells fabricated on such layers provided that its concentration is well controlled.
Design of a system to carry out Group VIA material and/or dopant material deposition is critical for the quality of the resulting absorber film, the efficiency of the solar cells, throughput, material utilization and cost of the process. The deposition flux from thermal sources tends to vary considerably during the deposition processes. Having the ability to measure and control deposition flux is critical for process stability. Therefore, there is need for new processes and tools to deposit such layers efficiently to form high quality, low cost CIGS type absorber layers for solar cells.